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John O | November 2018

Researchers demonstrate for first time that graphene enables clock rates in terahertz range

By Josh Perry, Editor


Scientists have long believed that graphene could be used to push electronics to higher clock rates, into the terahertz range, but there has been no proof of this capability, until recent research from Helmholtz Zentrum Dresden-Rossendorf (HZDR) and the University of Duisburg-Essen (UDE), in cooperation with the Max Planck Institute for Polymer Research (MPI-P), which demonstrates that graphene converts electronic signals from the gigahertz frequency to terahertz.


Graphene converts electronic signals with frequencies in the gigahertz range extremely efficiently into signals with several times higher frequency. (Juniks/HZDR)


According to a report from HZDR, not only were the researchers able to demonstrate frequency multiplication in a graphene monolayer for the first time but they also described the measurements using a model based on fundamental principles of thermodynamics.


“The long-awaited experimental proof of extremely efficient terahertz high harmonics generation in graphene has succeeded with the help of a trick: The researchers used graphene that contains many free electrons, which come from the interaction of graphene with the substrate onto which it is deposited, as well as with the ambient air,” the article explained.


It continued, “If these mobile electrons are excited by an oscillating electric field, they share their energy very quickly with the other electrons in graphene, which then react much like a heated fluid: From an electronic ‘liquid’, figuratively speaking, an electronic ‘vapor’ forms within the graphene. The change from the ‘liquid’ to the ‘vapor’ phase occurs within trillionths of a second and causes particularly rapid and strong changes in the conductivity of graphene. This is the key effect leading to efficient frequency multiplication.”


Researchers used electromagnetic pulses with frequencies between 300-680 gigahertz and converted them through graphene into pulses with up to seven times the initial frequency. They believe this will be a breakthrough for future nanoelectronics.


The research was recently published in Nature. The abstract read:


“Multiple optical harmonic generation—the multiplication of photon energy as a result of nonlinear interaction between light and matter—is a key technology in modern electronics and optoelectronics, because it allows the conversion of optical or electronic signals into signals with much higher frequency, and the generation of frequency combs. 


“Owing to the unique electronic band structure of graphene, which features massless Dirac fermions, it has been repeatedly predicted that optical harmonic generation in graphene should be particularly efficient at the technologically important terahertz frequencies. However, these predictions have yet to be confirmed experimentally under technologically relevant operation conditions.


“Here we report the generation of terahertz harmonics up to the seventh order in single-layer graphene at room temperature and under ambient conditions, driven by terahertz fields of only tens of kilovolts per centimetre, and with field conversion efficiencies in excess of 10−3, 10−4 and 10−5 for the third, fifth and seventh terahertz harmonics, respectively. These conversion efficiencies are remarkably high, given that the electromagnetic interaction occurs in a single atomic layer.


“The key to such extremely efficient generation of terahertz high harmonics in graphene is the collective thermal response of its background Dirac electrons to the driving terahertz fields. The terahertz harmonics, generated via hot Dirac fermion dynamics, were observed directly in the time domain as electromagnetic field oscillations at these newly synthesized higher frequencies. The effective nonlinear optical coefficients of graphene for the third, fifth and seventh harmonics exceed the respective nonlinear coefficients of typical solids by 7–18 orders of magnitude.


“Our results provide a direct pathway to highly efficient terahertz frequency synthesis using the present generation of graphene electronics, which operate at much lower fundamental frequencies of only a few hundreds of gigahertz.”

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